352 PROCEEDTNGS OF THE ACADEMY OP [1881. 



twelve to twenty arm openings to the species, and twice that 

 number of arms, two to each opening, given off in a similar 

 manner as in some species of Batocrinus and Eretmocrinus. The 

 arm openings are unequally divided among the rays, only species 

 with twenty openings having four to each ray. In species with a 

 less number, the posterior rays are stronger, and have always four 

 openings, except when there are only twelve openings in all. The 

 anterior ra}'- is generally stronger, or has at least as many arms as 

 the antero-lateral rays, which, as a rule, are the least developed, 

 and in most species have but two pairs of arms. Slight varia- 

 tions in the arm formula of the same species occur frequently in 

 this genus. Respiratory (?) pores close to the arm openings. 



Interradials two or three, rarely more, in two series ; the first 

 very large, frequently but not always extending to the third 

 primar}'^ radials ; the plates of the upper series small, placed within 

 the sinus between the rays, and hence are connecting with the 

 interradial dome plates. Anal area constructed very differently 

 from the interradial, composed of a large plate in line with the 

 first radials, but higher than those, with the upper sloping sides 

 longer. There are three plates in the second series, the middle 

 one larger with truncate upper side, succeeded by a row of two 

 to four similar plates, which form a longitudinal line, often 

 elevated into a ridge up to the anal aperture. The opening is 

 surrounded by one or two rows of plates, which at the upper side 

 lean against the central dome piece. It is a simple aperture 

 penetrating a somewhat thickened protuberance, and nearly 

 always situated and directed laterally. No interaxillary plates. 



Dome elevated, inflated, occasionally a little depressed toward 

 the middle. Plates large, centre plate and the first five radial 

 pieces larger than the proximal dome pieces, and frequently 

 extended into spines, which in some extreme cases attain a length 

 of three to four inches. There are other species with only a 

 central spine, and still others in which the respective plates 

 are only larger or more convex. The proximal plates connect 

 with the centre piece, they are verj' regularly arranged and 

 readily recognized, but otherwise not distinct from the rest of the 

 vault pieces. 



Column round, not strong, and composed alternately of much 

 larger and smaller segments ; central canal small. 



Geological Position, etc. — Dorycrinus is in America a strictly 



